The present invention relates in general to the thermal treatment of metals and in particular, to a new and useful method and apparatus for the slow cooling of steel stock, in particular, steel tubulars and bars.
Tubular products of low to medium carbon and alloy grades, ordered for machining applications, must have the proper thermal history to result in the development of a microstructure consisting of blocky ferrite and open lamellar pearlite. This desired condition is developed by controlling the cooling rate during transformation of the austenite phase into the ferrite and pearlite phases The temperatures of transformation from austenite to pro-eutectoid ferrite vary with chemical composition of the steel and the rate of cooling. The start of transformation may be as high as 1540.degree. F. for a straight low carbon steel, or as low as 1200.degree. F. for an alloy steel. Generally, the start of transformation is between these values for most grades considered for this process. Completion of transformation is also dependant upon chemical composition and cooling rates, and generally ranges between 1300.degree.-750.degree. F. Cooling rates of approximately 60.degree.-100.degree. F./hour are commonly employed industry standard practice for typical slow cool furnace facilities. However, rates as high as 200.degree. F./hour may provide acceptable results on some grades and applications. Cooling rates as low as 20.degree. F./hour or less may be necessary for certain grades and special applications. Generally, the slower the cooling rate to and through the transformation, the better the resultant microstructure.
In the absence of a slow cool furnace, typical cooling rates off the hot mill (approximately 600.degree.-1000.degree. F./hour) result in fine grain ferrite/pearlite microstructures for the carbon and low alloy steels, and in Widmanstatten or martensitic microstructures for the air hardenable grades. Such microstructures do not provide good machinability.
Additional detailed information concerning the thermal treatment of metals can be found in STEAM/ITS GENERATION AND USE, 39th EDITION, Published by Babcock and Wilcox.
Quality steel tubular producers have long supplied low to medium carbon and alloy steel tubing to the automotive industry and other end users in a condition considered favorable for subsequent machining on automatic screw machines and/or broaching operations.
One property vital to machinability is the microstructural characteristics of the product. Although order requirements and specifications do not generally stipulate a quantitative acceptance range on microstructure, (other than the limits on surface decarburization), these products, requiring special thermal handling are typically referred to as "blocky ferrite/pearlite" or "open lamellar pearlite".
Most producers take advantage of inherent heat associated with hot rolling, and retard cooling by transporting hot product into a slow cool furnace and by controlling the subsequent cooling rate so as to develop the desired microstructure. Babcock and Wilcox, for example, employed an off mill slow cool furnace at one of its plants for many years. Other plants utilized batch annealing furnaces which required annealing cycles of 32 to 40 hours.
Some plants lacked sufficient off mill space to allow the installation of a slow cool furnace. Therefore, alternate and more costly heat treatments and heat treat facilities are employed to generate the desired microstructural characteristics.
Manufacturing costs associated with the heat treatments, such as batch furnace annealing, pickling (scale removal by acid treatment) surface repair by cold drawing, and the like, add considerably to manufacturing costs.
A need remains for a simple and economical scheme for obtaining the desired microstructures in machinable steel stock.